1. Field of the Invention
The present invention relates to a method for manufacturing an SOI (Silicon on Insulator) substrate which has a monocrystal silicon layer on an insulating film and has attracted the attention as a substrate of a VLSI in the future, and particularly to a method, which enables manufacturing of the SOI substrate having a low defect density of the monocrystal silicon layer.
2. Description of the Related Art
FIGS. 9A-9C are cross sectional views, which show an example of a conventional manufacturing method of the SOI substrate, and specifically an SIMOX (Separation by Implanted Oxygen) method for manufacturing the SOI substrate in accordance with the steps.
In the SIMOX method, oxygen is ion-implanted at an implantation dose of about 2.times.10.sup.18 /cm.sup.2 into a silicon substrate by an accelerating voltage of about 200 keV, and then annealing or heat treatment is sufficiently applied to the substrate in gas mixture of Ar/O.sub.2 or N.sub.2 /O.sub.2 at a high temperature not less than 1300.degree. C., so that a silicon oxide layer (SiO.sub.2 film), i.e., a buried insulating film is directly formed in the silicon substrate. A conventional manufacturing method of the SOI substrate will be described below.
First, a silicon substrate 1 shown in FIG. 9A is heated to 500.degree.-600.degree. C., In this state, as shown in FIG. 9B, oxygen ion 2 is implanted into silicon substrate 1 at the implantation dose of about 2.0.times.10.sup.18 /cm.sup.2 by the accelerating voltage of 200 keV. Silicon substrate 1 and the oxygen which is directly implanted thereinto react on each other to form a buried silicon oxide film 3.
FIG. 12 shows a density profile of oxygen in the silicon substrate in the cases that oxygen ion is implanted at various implantation doses by the accelerating voltage of 200 keV. With reference to FIG. 12, if the implantation dose of the oxygen ion is low, the oxygen shows a Guassian distribution in the silicon substrate. However, if the oxygen ion implantation dose is higher than a critical implantation dose of 1.35.times.10.sup.18 /cm.sup.2, an oxygen concentration near an implantation peak exceeds a stoichiometric concentration of SiO.sub.2, i.e., 4.4.times.10.sup.22 /cm.sup.3, so that an SiO.sub.2 film is formed. Excessive oxygen diffuses toward a base of the distribution, and reacts on the silicon substrate to form SiO.sub.2, so that an interface between the SiO.sub.2 film and the silicon substrate becomes steep. This reaction can be expressed in the following formula. EQU xSi+2O.sub.i .fwdarw.SiO.sub.2 +(x-1) Si.sub.i ( 1)
wherein O.sub.i is interstitial oxygen, and Si.sub.i is interstitial silicon. In this reaction, the interstitial silicon is emitted for relieving increase of a volume (by 2.25 times) caused by the formation of the SiO.sub.2 layer. This interstitial silicon is absorbed into a surface of the silicon substrate which forms sink. However, as the implantation dose of oxygen ion increases, a number of the generated interstitial silicon increases, and the excessive interstitial silicon will be coupled together to form defect in a residual silicon layer 4 (FIG. 9B).
Residual silicon layer 4 is damaged by the ion implantation, and also has a high oxygen concentration and the above described defect. Therefore, layer 4 requires heat treatment, for example, in an atmosphere of Ar/O.sub.2 at a temperature of 1300.degree. C. for 5 hours, in order to restore crystallinity of the residual silicon layer 4. In a conventional heat treatment step, as shown in FIG. 10, silicon substrate 1 is heated at a constant temperature-rise rate until it reaches a holding temperature (1300.degree. C.). FIG. 11 shows a time chart of these typical heat treatment step. In a flow of the typical heat treatment, the temperature is raised at the temperature-rise rate of, e.g., 20.degree. C./min. to 1300.degree. C., and then is held in the atmosphere of argon (Ar) and a very small amount of oxygen (O.sub.2) mixed therein for 6 hours. Thereafter, it is cooled in a furnace for about two hours nearly to a room temperature. By the heat treatment, residual silicon layer 4 is reformed into a monocrystal silicon layer (SOI layer) 5 (FIG. 9C). Meanwhile, in accordance with the progress of the reaction of formula (1), a thickness of SiO.sub.2 layer 3 increases, and silicon oxide particles functioning as oxygen precipitate in the residual silicon layer 3 are captured into SiO.sub.2 layer 3 due to melting or coupling, resulting in the steep SiO.sub.2 layer 3 having only one interface. Growth or dissolution of silicon oxide particles, i.e., the oxygen precipitate depends on a critical radius r.sub.o expressed by the following equation. EQU r.sub.o =(2.sigma./.DELTA.H.sub.V).multidot.(T.sub.E /(T.sub.E -T))(2)
.DELTA.H.sub.V : enthalpy of formation of the SiO.sub.2 phase per unit volume PA1 .sigma.: Si/SiO.sub.2 interface energy per unit surface PA1 T.sub.E : equilibrium temperature PA1 T: heat treatment temperature
In accordance with the equation (2), as heat treatment temperature T increases, a value of (T.sub.E -T) decreases, so that critical radius r.sub.o increases. Therefore, as heat treatment temperature T increases, the precipitate having a radius smaller than critical radius r.sub.o dissolves. Also, the heat treatment generates a large amount of interstitial silicon, a part of which is absorbed into the surface. The excess interstitial silicon, however, is used for generation and growth of the defect, and ultimately the defect is stabilized in a form of a threading dislocation 6 which fixedly penetrates a surface 51 of monocrystal silicon layer 5 and an interface between monocrystal silicon layer 5 and silicon oxide film 3, as shown in FIG. 9C. Threading dislocation 6 remains in monocrystal silicon layer 5 to deteriorate the crystal quality thereof. A mechanism of formation of such dislocation is disclosed, for example, in J. Stoemenos et al., J. Appl. Phys., Vol. 69, NO. 2, Jan. 15, 1991, pp. 793-802.
The density of the threading dislocation depends on conditions for implanting the oxygen ion. As shown in FIG. 13, the higher implantation dose of oxygen ion and the lower accelerating voltage tend to increase the dislocation density. A multiple ion implanting (multi-step implanting) method utilizing the correlativity between the implantation dose of oxygen ion and the dislocation density (defect density) has been developed and reported in D. Hill et al., J. Appl. Phys., Vol. 63, No. 10, May 15, 1988, pp. 4933-4936. In this method, the oxygen ion is implanted at-the implantation dose (in a range from 0.5 to 1.times.10.sup.18 /cm.sup.2) lower than the conventional dose in order to reduce the dislocation density, and then annealing is conducted for restoring crystallinity and growing the SiO.sub.2. Thereafter, the implantation of oxygen ion and the annealling are repeated for few times in order to obtain a predetermined implantation dose. This method can produce the good SOI substrate, in which the interface between the monocrystal silicon layer and the silicon oxide film is very steep, and also the dislocation density of the monocrystal silicon layer is lower than 10.sup.3 /cm.sup.2. This method, however, requires complicated processes, and thus is not suitable to a commercial mass production.
Another method for reducing the dislocation density has been proposed in M. K. EL-Ghor et al., Appl. Phys., Lett., Vol. 57, No. 2, Jul. 9, 1990, pp 156-158. In this method, cavities are formed in the SOI layer at a high density during the implantation of oxygen ion. The cavities act as the sink of the interstitial silicon and thus reduce the dislocation density.
Since conventional manufacturing method of the SOI substrate is conducted as shown in FIGS. 9A-9C, the threading dislocation remains in the SOI layer at a density of about 10.sup.8 /cm.sup.2. If the threading dislocation remains, and thus MOS transistors are formed in the SOI layer having deteriorated crystal quality, the defect and/or impurity are captured into the gate oxide film when the gate oxide film is formed, which reduces a resistance against electric field of the gate oxide film. Also the defect existing in the depleted layer causes a generation current, which increases a power consumption. As described above, if the MOS transistors are formed in the SOI layer containing the residual threading dislocation, characteristics of the device deteriorate.
Although the multiple ion implanting method provides the good SOI substrate having the low dislocation density, it requires the complicated processes, and is not suitable to the commercial mass production.